On first glance, the Dogbane Leaf Beetle is a stunning metallic blue-green, that clearly stands out on the light-green Dogbane leaves it feeds on. However, when you look closer, you can see the highlights of the elytra and the pronotum change in colour depending on the viewing angle and the angle of the light source. Shades of copper, orange, yellow, blue and even a shadowy black emerge as the beetle moves around a leaf. How does this iridescent colour shift happen?
(The beetle above was photographed with reflected electronic flash while climbing up the walls in a white box)
The best open-access paper I could find on beetle colour was Gold bugs and beyond: a review of iridescence and structural colour mechanisms in beetles (Coleoptera) by Seago, Ainsley E. et al. The paper reviews all the different iridescence mechanisms known in Coleoptera, from both the entomological and optical (i.e. physics) literature. (From now on, the location of information made within the paper will be shown parentheses.)
Before looking at the mechanism that applies to our little metallic friend, I should define exactly what ‘iridescence’ means. The paper’s definition of iridescence is based on that of Mason (1927): ‘iridescence has for its main characteristic a change in the hue of the object exhibiting it as the angle of vision is varied’ (1.1), which is close to my observations in the first paragraph. The paper provides three mechanisms that can cause iridescence–multilayer reflectors, three-dimensional photonic crystals and diffraction gratings. For the family Chrysomelidae, in which the Dogbane Leaf Beetle resides, the common colour mechanism is multilayer reflectors (2.3).
How are multilayer reflectors developed? When a beetle grub undergoes metamorphosis in the pupal stage, the epidermis secretes parallel layers of chitin to form the exoskeleton. These layers may be of different thicknesses and can be separated by other material so once they harden they refract light differently.
(Visible light spectrum image from Wikipedia)
As I understand it, the optical properties of thick layers allow long wavelengths, like orange or red, to pass through while thinner layers pass shorter wavelengths such as blue. When we vary our viewing angle or if the angle of the light source changes, we are lengthening and shortening the wavelengths of light in different areas. The refracted light can merge (e.g. green + yellow = orange) and change the colours that are reflected back. (2.1) (See fig 2 online to for the different types of multilayer reflectors.) The black areas in the reflections are probably caused by the black camera, capturing its own reflection.
So why have these bright eye-catching iridescent metallic colours anyway? What evolutionary function does this have? Considering their iridescence is primarily green, the most obvious answer is crypsis, the ability to remain unobserved. However to my eye, the beetle is highly visible even in green leafy surroundings, but that may not the case for potential predators. The fact that these beetles are often seen brazenly exposed on leaf surfaces, sometimes even when mating (see previous post) shows that there may be an aposematic (warning) element to their coloring. Their food plants are Milkweed and Dogbane, both of which have sap containing toxic cardenolides. Dogbane beetles can sequester (store) cardenolides and are known to release them from pronotal and elytral glands, and this may discourage some predators and parasites.
Dobler, S., D. Daloze, J. Pasteels. 1998. Sequestration of plant compounds in a leaf beetle’s defensive secretion: cardenolides in Chrysochus. Chemoecology, 8: 111-118.
Seago, A. E., Brady, P., Vigneron, J.-P., & Schultz, T. D. (2009). Gold bugs and beyond: a review of iridescence and structural colour mechanisms in beetles (Coleoptera). Journal of the Royal Society Interface, 6(Suppl 2), S165–S184. doi:10.1098/rsif.2008.0354.focus
Animal Diversity Web — Chrysochus auratus